Method and device for processing video signals for presentation on a display and corresponding computer program product

ABSTRACT

An RGB digital video signal destined to be displayed on a display such as a liquid crystal display (LCD) is converted from the RGB color space to the YUV color space. The signal converted into the YUV color space is subjected to at least a processing operation selected among a sub-sampling operation ( 24 ) and a data compression operation ( 26 ). The signal is then stored in a memory and the signal read from said memory ( 12 ) is then subjected to at least a return operation ( 28, 30 ) complementary to the aforesaid processing operation ( 24, 26 ). The signal subjected to the aforesaid return operation is lastly reconverted from the YUV color space to the RGB color space, thus being susceptible to be displayed on the display.

FIELD OF THE INVENTION

The present invention relates in general to liquid crystal displays,commonly denominated with the acronym LCD.

Especially in the color LCD version, the devices in question are appliedin the most modern PDA (Personal Digital Assistant) devices and in thirdgeneration multimedia cell phones, with advanced video and graphicscapabilities, whose use is destined to become ever more widespread overthe next few years.

DESCRIPTION OF THE RELATED ART

FIG. 1 in the accompanying drawings shows—in general terms—thearchitecture currently used for driving an LCD display, designated withthe reference number 10.

In the diagram of FIG. 1, the display 10, having characteristics whichcan be considered wholly known insofar as the present context isconcerned, carries associated therewith a memory 12 destined to containthe information about the image to be shown on the display 10. Thememory 12, also known as picture memory, is read periodically and theinformation contained therein is transferred to the display 10, whichdisplays them. To display an image on the display 10, therefore, it isnecessary to record it on the memory 12, wherefrom it will be read andtransferred to the display.

The memory 12 becomes necessary because, due to the physical propertiesof the fluid composing the liquid crystal cell, the content of eachindividual cell of the display 10 must periodically be overwritten withhigh frequency. The image to be display is then written in the memory 12with a certain frequency f_(write) and thence read and transferred tothe display 10 with a frequency f_(read) that is much greater thanf_(write), with f_(read) being the so-called display refresh frequency.Typically, f_(read) has a value that is 5 to 10 times greater thanf_(write). For instance, f_(read) can be 70 Hz, whilst f_(write) can be15 Hz, if an MPEG-4 video stream is to be display.

The image is, in fact, a rectangular matrix of N samples called pixels(picture elements), each of which is expressed by means of threechromatic components, in other words through the intensity of the redcomponent (R stands for “red”), of the green component (G stands for“green”) and of the blue component (B stands for “blue”). In this way,the resulting color “c” of the pixel is ideally given by therelationship:c=R+G+B

In a digital application, each of the three components is quantified ona certain number of levels and generally expressed with a precision ofeight bit or less.

For a general overview of the RGB format in the application scopeconsidered herein, reference can usefully be made to the work by J. L.Mitchell, W. B. Pennebaker, C. E. Fogg and D. J. LeGall, “Mpeg videocompression standard”, Chapman & Hall, 1997.

If each of the three components is expressed with eight bit precision,graphics are known as “true color”, and each pixel thus occupies amemory space of 24 bits. In this case, possible color combinations are2²⁴, i.e. more than 16 million.

In so-called “high color” graphics, available pixels are reduced to 16and distributed as follows: 5-pixel precision for red, 6-pixel for greenand 5-pixel for blue for a total of 2¹⁶ possible combinations, equal to65,536.

Hence, in the case of a “true color” system, the memory required tocontain an image with a width of W pixels and a height of H pixels isequal to W.H.24 bit=3.W.H bytes. Considering as typical a display ofQCIF dimensions, so that W=176 and H=144, the memory required thereforeis equal to 76,032 bytes; in the case of high color graphics, instead,it is equal to 176.144.2=50,688 bytes.

To complete the description of FIG. 1, the reference 14 indicates adigital/analog converter, of a type globally known: for instance, it canbe a PWM DAC converter destined to convert the digital signals read fromthe memory 12 into electrical signals allowing the actual presentationof the pixels on the display 10.

Structure and characteristics of the converter 14 must be consideredglobally known and, in any case, not relevant in themselves for purposesof understanding the present invention.

The assembly comprised of the memory 12 and the converter 14 is usuallyreferred to as a “driver” associated with the display (LCD driver).

The diagram of FIG. 2 refers to a known structure of LCD driver,corresponding in particular to the device HD66766 manufactured by thecompany Hitachi (see in this regard the related data sheet, Rev 001,August 2001).

The solution shown in FIG. 2 allows to drive a display (not shown inFIG. 2) operating in “high color” mode.

For this purpose the incoming RGB signal is subjected, in a moduledesignated as 12 a, to a so-called dithering operation which aims toreduce the number of colors of the incoming image to the requiredprecision. The RGB signal subjected to dithering is stored in the memory12 to whose output is associated a module 12 b.

Here, before they are transferred to the display, the signals read fromthe memory 12 are subjected to a “bit stuffing” operation aimed atrestoring the bits lost during the dithering phase by attributing avalue 0 to them.

OBJECT AND SUMMARY OF THE INVENTION

Although the aforementioned solutions according to the prior art areundoubtedly functional, the need remains to achieve additionalimprovements regarding the reduction of the memory of the driverassociated to the LCD especially in view of a consequent reduction inoccupied silicon area, of dissipated power and of the cost of thedevice.

An object of the present invention is to provide an enhanced solution inthis respect.

According to the present invention, this aim is achieved thanks to amethod having the characteristics specifically set out in theaccompanying claims. The invention pertains to the corresponding device,as well as to the corresponding computer program product loadable intothe memory of a computer and including software code portions forimplementing the method according to the invention when the product isrun on a digital processor.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The invention shall now be described, purely by way of non limitingexample, with reference to the accompanying drawings, in which:

FIGS. 1 and 2, the latter one specifically representative of a prior artsolution, have already been described above,

FIG. 3 represents, in the form of a function block diagram, a possibleembodiment of the solution according to the invention

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The function block diagram of FIG. 3 should be read in co-ordinationwith the diagrams of FIGS. 1 and 2.

The blocks 20 through 26 of the diagram of FIG. 3 refer to treatmentoperations/modules which intervene on the incoming RGB signal in view ofits storage in the memory 12. The blocks designated with the references28 through 36 instead indicate operations/modules intervening on thesignal read from the memory 12 in view of its transfer to the display 10(typically via the converter 14).

Next to the various blocks of FIG. 3 are indicated the lengths of thestrings of bits present respectively at the input and at the output ofeach block.

It is thus readily apparent that the incoming RGB signal, arranged on 12or 24 bits, is reduced to an 8-bit format in view of its storage in thememory 12. Starting from the signal read from the memory 12 (arranged on8 bits), an output RGB signal on 12–24 bit is then (re)generated.

As stated, the object of the invention is to minimize the silicon arearequired by the picture memory 12, whilst maintaining a high videoquality of the image, as well as limited electrical power consumption.

Preferably, the first operation performed on the incoming RGB signal isa bit stuffing operation (module 20), serving the purpose of bringingsample precision to the required value. The next part of the processingchain is conceived to work on true color graphics, with 8-bit precisionfor each chromatic component. This allows the driver to drive any kindof display, regardless of the number of colors it supports.

If the incoming image then has a precision that is lower than 24 bitsper pixel (for example, 12 bits per pixel), the precision is increasedto 24. This is done by setting to zero the most significant bits (MSB)or the least significant bits (LSB) for each chromatic component. Forinstance, if the incoming R, G or B components consist of 4, 5, 6 bits,respectively 4, 3, 2 null bits are inserted in MSB (or LSB) positiondepending on the selected solution.

The next module in the system, designated with the reference number 22,operates a conversion of the chromatic space of the pixels from thetraditional RGB space (in the red, green and blue components) to the YUVspace (of the luminance and chrominance components).

For a general illustration of the characteristics of the YUV chromaticspace, reference can usefully be made to the previously mentioned workby J. L. Mitchell et al.

The operation can be simply expressed by means of a matrix relationshipof the following kind: $\begin{bmatrix}Y \\U \\V\end{bmatrix} \equiv {\begin{bmatrix}{yr} & {yg} & {yb} \\{ur} & {ug} & {ub} \\{vr} & {vg} & {vb}\end{bmatrix} \times \begin{bmatrix}R \\G \\B\end{bmatrix}}$

The elements of the conversion matrix of the color space can beexpressed as short sums of powers of two, reducing the chromaticconversion to simple elementary shifting and summing operations,achievable at the hardware level with extreme efficiency.

This operation does not change the precision of the pixels, whichremains 24 bits in total.

The conversion from the color space RGB to the color space YUV takesinto account the physical characteristics of the human eye, which isvery sensitive to the luminance component (Y), but not very sensitive tothe chrominance components (U and V). This allows to filter chrominance,reducing the number of samples and thus effecting a first reduction ofthe memory taken up by the image.

Considering the image to be displayed as composed by three matrices Y,U, and V each of which has a dimension W.H, it is possible to proceedwith a sub-sampling, and in particular with a horizontal sub-samplingwhich operates on the U and V matrices, reducing their dimensions toW/2.H, leaving the Y matrix unchanged. The overall effect is a minimaldeterioration in image quality, certainly not perceptible to the humaneye. In fact, the filtering can be expressed as:${y(n)} = {\sum\limits_{n}\;{{a(n)} \cdot {x(n)}}}$

where the summation extends for n ranging from −N/2 to N/2.

Specifically, y(n) represents the filtered pixel, whilst x(n) is thesource pixel. N is the number of adjacent samples necessary to performthe filtering, whilst a(n) designates the numerical coefficients of thefilter. Taking into account the characteristics of the application, thecomplexity of the block can be minimized, setting N=2 and causing thecoefficients to be reducible to a short sum of powers of two.

The data deriving from the sub-sampling operation performed in themodule 24 are subjected, in the module designated as 26, to a datacompression operation. This is in preferred fashion a data compressionoperation carried out operating differently on the luminance andchrominance components.

In particularly preferred fashion, the adopted solution is the oneemployed in the Rempeg 50 encoding/decoding device, manufactured by thesame Applicant (in this regard, one can usefully consult the document R.Burger, D. Pau, “Rempeg 50 encoder/decoder. User manual”, March 1999).

When it operates on luminance, the compression function receives at itsinput 16 bytes, corresponding to 16 samples and returns 9 at its output,thereby obtaining a constant compression factor of 9/16. When itoperates on the chrominance, the algorithm receives at its input 8bytes, corresponding to 8 samples and returns 4 at its output, therebyobtaining a constant compression factor of 1/2.

In strict terms, the compression solution described herein is of the“lossy” type, i.e. with information losses. The tests conducted by theApplicant, however, demonstrate that, in most envisioned applications,image quality deterioration is in fact negligible, hence not perceptibleto the human eye.

It will be further appreciated that, though the execution of both ispreferred, the solution according to the invention does not necessarilyrequire the signal converted into the YUV format to be subjected both tosub-sampling and to compression.

Thanks to the transformation operations conducted in the modules 20through 26 described above, the quantity of memory 12 that needs to beincluded in the LCD driver is very small.

In particular, the occupation of the entire image can be reduced toabout 35%, thus saving as much as 65% of memory. All with negligibledeterioration in quality both due to the perception characteristics ofthe human eye, and to the physical response characteristics of theliquid crystal display.

Therefore, a true color image can be stored in a space of 26,928 bytes,as opposed to the 76,032 bytes required by the other known solutions.

In regard to the treatment operations performed on the signal extractedfrom the memory 12, the modules designated as 28, 30 and 32 performfunctions that are essentially complementary to the functions performed,respectively and in order, by the modules designated as 26, 24 and 22.

Thus the module 28 essentially performs a decompression that iscomplementary to the compression function performed in the module 26.

If, for example, for the module 26 the aforementioned Rempeg solutionwas adopted, the module 28 uses the Rempeg decompression function.According to this solution, when it decodes luminance, the module 28receives 9 bytes at its input and returns 16, corresponding 16 samples.When it decodes chrominance, the module 28 receives 4 bytes at its inputand returns 8, corresponding to 7 samples.

The module 30 instead is a horizontal super-sampling module which, by aninverse filtering operation on the chrominance components, converts the4:2:2 format to the 4:4:4 format, returning the U and V matrices to thedimension W.H.

The modules 28 and 30 thus perform a returning operation, complementaryto the operation or operations carried out by the modules 24 and 26.

Lastly, the module 32 inverts the matrix operation of color spacetransformation performed by the module 22, returning the signal fed atits input from the color space YUV to the initial color space RGB, witha precision of 8 bits for each of the 3 chromatic components.

The presence, in the diagram of FIG. 3, of the modules 34 and 36 isaimed at taking into account the fact that the display is driven bymeans of analog signals, so that the RGB values of the pixels must bemade to pass through a digital/analog converter (DAC), such as theconverter 14 of FIG. 1, whose precision is less than or equal to 8 bits.If it is less than 8, it is necessary to reduce the precision of the RGBsamples from 8 bits to the required value through a bit manipulationoperation.

For example, the following techniques can be used:

-   -   truncating the least significant bits,    -   approximation of the least significant bits to the closest        integer, and    -   sum of pseudo-random noise followed by truncation.

These solution are substantially equivalent to each other, so the choiceof one or the other is dictated by specific construction requirements.

In the case of 12, 16 and 18 bit inputs, the inverse bit stuffingfunction is implemented (in the module designated as 32). This operationcan indifferently be operated eliminating the most significant bits(MSB) or the least significant bits (LSB) for each chromatic component,inserted as described above with reference to the module 20. Forexample, if for each R, G or B component the input consists of 4, 5 6bits, then—respectively—4, 3 or 2 bits are removed in MSB (or LSB)position, depending on the selected solution.

The module 36, lastly, is destined to perform a range correctionoperation, aimed at correcting the intrinsic non-linearities of thedisplay response by means of an appropriate re-mapping of the colors.

This function is performed (according to criteria known in themselves)by means of a look-up table (LUT).

Naturally, without altering the principle of the invention, theconstruction details and the embodiments may vary widely from what isdescribed and illustrated herein, without thereby departing from thescope of the present invention.

In particular, it will be appreciated that the solution according to theinvention is susceptible to be embodied advantageously by means of adedicated processor. Alternatively, the solution according to theinvention is susceptible to be embodied using a general purposeprocessor, appropriately programmed by means of a computer programproduct loadable into the memory of such a digital processor andincluding software code portions for implementing the method accordingto the invention when the computer program product is run on a digitalprocessor.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

1. Method for processing an RGB digital video signal to be displayed ona display with the use of a memory, the method comprising the operationsof: converting said digital video signal from an RGB color space to aYUV color space to obtain a YUV video signal; subjecting the YUV videosignal to a processing operation selected from among a sub-samplingoperation and a data compression operation to obtain a processed videosignal; storing in said memory said processed video signal; reading fromsaid memory the signal stored therein; subjecting the processed videosignal read from said memory to a return operation that is complementaryto said processing operation, thereby obtaining a re-processed videosignal; and reconverting the re-processed video signal from the YUVcolor space to the RGB color space, to obtain a reconverted RGB digitalvideo signal, wherein at least one among said converting andreconverting operations is performed as a matrix conversion usingcoefficients expressible as sums of powers of 2, so that said at leastone among said converting and reconverting operations is performed usinga series of elementary shifting and summing operations.
 2. Method asclaimed in claim 1, wherein said display is a liquid crystal display. 3.Method as claimed in claim 1 wherein the operation of subjecting the YUVvideo signal to the processing operation includes both of saidoperations of sub-sampling and data compression, and said returnoperation comprises both a data decompression operation and asuper-sampling operation.
 4. Method as claimed in claim 1 wherein saidsub-sampling operation operates on chrominance matrices of said YUVvideo signal leaving unchanged a luminance matrix of said YUV videosignal.
 5. Method as claimed in claim 1 wherein said sub-samplingoperation is a horizontal sub-sampling operation.
 6. Method as claimedin claim 1 wherein said data compression operation operates indifferentiated fashion on components of luminance and chrominance ofsaid YUV video signal.
 7. Method as claimed in claim 1, furthercomprising a bit stuffing operation performed on said RGB digital videosignal before conversion into said YUV color space.
 8. Method as claimedin claim 7, further comprising a bit rounding operation performed onsaid reconverted RGB signal before displaying said reconverted RGBsignal on said display.
 9. Method as claimed in claim 1, furthercomprising: displaying said reconverted RGB signal on said display; anda range correction operation by re-mapping said reconverted RGB signalin view of its display on said display.
 10. Device for processing an RGBdigital video signal for its display on a display, the devicecomprising: a conversion module for converting said digital video signalfrom an RGB color space to a YUV color space to obtain a YUV videosignal; a processing module for subjecting the YUV video signal to aprocessing operation selected among a sub-sampling operation and a datacompression operation, thereby obtaining a processed video signal; amemory for storing said processed video signal; a return module forreceiving the processed video signal stored in said memory andsubjecting the processed video signal received from said memory to areturn operation, complementary to said processing operation, therebyobtaining a reprocessed video signal; and a re-conversion module forre-converting the reprocessed video signal from the YUV color space tothe RGB color space, to obtain a reconverted RGB digital video signals,wherein at least one of said conversion module and said re-conversionmodule is configured to carry out a matrix conversion using coefficientsexpressible as sums of powers of 2, so at least one among saidconverting and re-converting is performed by using a series ofelementary shifting and summing operations.
 11. Device as claimed inclaim 10, wherein the device includes the display and the display is aliquid crystal display.
 12. Device as claimed in claim 10 wherein theprocessing module includes both a sub-sampling module and a datacompression module, and the return module includes both a datadecompression module and a super-sampling module.
 13. Device as claimedin claim 10 wherein said processing module is configured to carry outsaid sub-sampling operation by operating on chrominance matrices of saidYUV video signal, leaving unaltered a luminance matrix of said YUV videosignal.
 14. Device as claimed in claim 10 wherein said processing moduleis configured to carry out said sub-sampling operation as a horizontalsub-sampling operation.
 15. Device as claimed in claim 10 wherein saidprocessing module is configured to carry out said data compressionoperation operating in differentiated fashion on a luminance componentand on a chrominance component of said YUV video signal.
 16. Device asclaimed in claim 10, further comprising a bit stuffing module capable ofoperating on said RGB digital video signal before its conversion intoYUV color space.
 17. Device as claimed in claim 16, further comprising abit rounding module capable of operating on said reconverted RGB signalbefore it is displayed on said display.
 18. Device as claimed in claim10, further comprising a range correction module for carrying out are-mapping of said reconverted RGB signal in view of its display on saiddisplay.
 19. Computer program product loadable into a memory of adigital processor and including software code portions for implementing,when said computer product is run on a digital processor, a method forprocessing an RGB digital video signal to be displayed on a display, themethod comprising: converting said digital video signal from an RGBcolor space to a YUV color space to obtain a YUV video signal;subjecting the YUV video signal to a processing operation selected fromamong a sub-sampling operation and a data compression operation toobtain a processed video signal; storing in said memory said processedvideo signal; reading from said memory the processed video signal storedtherein; subjecting the processed video signal read from said memory toa return operation that is complementary to said processing operation,thereby obtaining a re-processed video signal; and reconverting there-processed video signal from the YUV color space to the RGB colorspace, to obtain a reconverted RGB digital video signal, wherein atleast one among said converting and reconverting operations is performedas a matrix conversion using coefficients expressible as sums of powersof 2, so that said at least one among said converting and reconvertingoperations is performed using a series of elementary shifting andsumming operations.
 20. The computer program product of claim 19 whereinthe operation of subjecting the YUV video signal to the processingoperation includes both of said operations of sub-sampling and datacompression, and said return operation comprises both a datadecompression operation and a super-sampling operation.
 21. The computerprogram product of claim 19 wherein said sub-sampling operation operateson chrominance matrices of said YUV video signal leaving unchanged aluminance matrix of said YUV video signal.
 22. The computer programproduct of claim 19 wherein said sub-sampling operation is a horizontalsub-sampling operation.
 23. The computer program product of claim 19wherein said data compression operation operates in differentiatedfashion on components of luminance and chrominance of said YUV videosignal.
 24. The computer program product of claim 19, further comprisinga bit stuffing operation performed on said RGB digital video signalbefore conversion into said YUV color space.
 25. The computer programproduct of claim 24, further comprising a bit rounding operationperformed on said reconverted RGB signal before displaying saidreconverted RGB signal on said display.
 26. The computer program productof claim 19, wherein said display is a liquid crystal display.
 27. Thecomputer program product of claim 19, further comprising: displayingsaid reconverted RGB signal on said display; and a range correctionoperation by re-mapping said reconverted RGB signal in view of itsdisplay on said display.